In a digital circuit, the removal of noise components from the input signal is essential for the correct operation of the circuit. Therefore, heretofore, a filter has been inserted in the input circuit of the digital circuit, to eliminate noise components, however, it is not always possible to completely remove all the noise components.
For example, in the case where an RC low-pass analog filter is inserted in the input circuit, high-frequency noise components in the input signal may be eliminated. However, it is impossible for the filter to remove low-frequency noise components or noise components long in duration.